1. Field of the Invention
The present invention relates to an image recording apparatus provided with plural image forming units.
2. Description of the Prior Art
There are already known various apparatus for recording an image on a paper sheet, such as a copier for reproducing the image of an original document onto a recording sheet by means of a photosensitive member, or printers for recording image information in the form of electric signals onto recording sheets, for example, impact printers (typefont or wire-dot printers) or non-impact printers (thermal, ink jet or laser beam printers).
In recent years higher performance is required for these image recording apparatus for achieving a higher efficiency in the office. Particularly there is required and is being achieved a higher output speed, namely an increase of the number of sheets to be processed per unit time.
However each process has its specific speed, and a higher speed involves various problems which require tremendous developmental efforts to overcome and which tend to require an economically unjustifiable complicated and large apparatus.
In the example of a copier, only one copy can be obtained from a scanning of the original, as a latent image is formed on the photosensitive member by focusing the original image thereon, then transformed into a visible image and transferred onto the recording sheet. Consequently a higher speed can be achieved by only a higher machine speed, which gives rise to insufficient sensitivity of the photosensitive member and vibration, noise and reduced service life of the mechanical parts. On the other hand, if the lamp for illuminating the original is used at a higher light intensity, the heat generated by the lamp will raise the temperature of the apparatus. In this manner there will appear numerous problems that are difficult to resolve in a complete manner, and the number of such problems increase logarithmically with the increase of the speed. Consequently an upper limit speed is defined in consideration of the balance with the advantages obtained by the higher speed.
On the other hand, there exists a demand for recording information on both sides of the recording sheet, as in the ordinary printing, for the purpose of saving resources and filing space. In order to meet such demand there is already proposed a method of stacking the recording sheets after information recording on the first sides thereof, and feeding said sheets again for information recording on the second sides thereof. This method is acceptable in case of producing plural copies, each having the same recording, but is quite inefficient in case of two-sided copies each having different recording. More specifically, in case of copying different pages 1, 2, 3, 4, . . . , there is required to at first record the odd pages 1, 3, 5, . . . on the sheets and then to feed said sheets again to record the even pages 2, 4, 6, . . . on the opposite sides thereof. However, if there occurs a trouble in said re-feeding of the sheets such as sheet jamming or simultaneous feeding of two sheets, the entire recording operation has to be repeated anew since the combination of the record on the first and second sides of a sheet becomes different. Such error can be avoided by information recording in succession on the first and second sides of each sheet, but such procedure will be extremely inefficient because of the cumbersome operation required for the sheet feeding.
As a part of the higher performance recording, there exists a demand for recording two information on a side of the recording sheet. Particularly in response to the recent popularity of colored information in various fields, there is required, in contrast to the conventional black records, to add a red record for example for the title of a document for greater legibility. Such mixed-color recording is possible in the conventional printer utilizing ink ribbon, but has not been realized in the electrostatic copier or laser beam printer for ordinary office use because the apparatus inevitably becomes quite bulky. A simpler method to achieve this objective with the conventional apparatus consists of at first recording the information with black color and then recording another information with red color on the same side by replacing the image developing unit in the apparatus for black color with another unit for red color, but such method is practically unacceptable because of its cumbersomeness.